Re: [PATCH] powerpc: Fix initrd corruption with relative jump labels

[Roman Bolshakov]

On Mon, Jun 14, 2021 at 11:14:40PM +1000, Michael Ellerman wrote:
> Commit b0b3b2c78ec0 ("powerpc: Switch to relative jump labels") switched
> us to using relative jump labels. That involves changing the code,
> target and key members in struct jump_entry to be relative to the
> address of the jump_entry, rather than absolute addresses.
> 
> We have two static inlines that create a struct jump_entry,
> arch_static_branch() and arch_static_branch_jump(), as well as an asm
> macro ARCH_STATIC_BRANCH, which is used by the pseries-only hypervisor
> tracing code.
> 
> Unfortunately we missed updating the key to be a relative reference in
> ARCH_STATIC_BRANCH.
> 
> That causes a pseries kernel to have a handful of jump_entry structs
> with bad key values. Instead of being a relative reference they instead
> hold the full address of the key.
> 
> However the code doesn't expect that, it still adds the key value to the
> address of the jump_entry (see jump_entry_key()) expecting to get a
> pointer to a key somewhere in kernel data.
> 
> The table of jump_entry structs sits in rodata, which comes after the
> kernel text. In a typical build this will be somewhere around 15MB. The
> address of the key will be somewhere in data, typically around 20MB.
> Adding the two values together gets us a pointer somewhere around 45MB.
> 
> We then call static_key_set_entries() with that bad pointer and modify
> some members of the struct static_key we think we are pointing at.
> 
> A pseries kernel is typically ~30MB in size, so writing to ~45MB won't
> corrupt the kernel itself. However if we're booting with an initrd,
> depending on the size and exact location of the initrd, we can corrupt
> the initrd. Depending on how exactly we corrupt the initrd it can either
> cause the system to not boot, or just corrupt one of the files in the
> initrd.
> 
> The fix is simply to make the key value relative to the jump_entry
> struct in the ARCH_STATIC_BRANCH macro.
> 
> Fixes: b0b3b2c78ec0 ("powerpc: Switch to relative jump labels")
> Reported-by: Anastasia Kovaleva <a.kovaleva@xxxxxxxxx>
> Reported-by: Roman Bolshakov <r.bolshakov@xxxxxxxxx>
> Reported-by: Greg Kurz <groug@xxxxxxxx>
> Reported-by: Daniel Axtens <dja@xxxxxxxxxx>
> Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> ---
>  arch/powerpc/include/asm/jump_label.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/include/asm/jump_label.h b/arch/powerpc/include/asm/jump_label.h
> index 2d5c6bec2b4f..93ce3ec25387 100644
> --- a/arch/powerpc/include/asm/jump_label.h
> +++ b/arch/powerpc/include/asm/jump_label.h
> @@ -50,7 +50,7 @@ static __always_inline bool arch_static_branch_jump(struct static_key *key, bool
>  1098:	nop;					\
>  	.pushsection __jump_table, "aw";	\
>  	.long 1098b - ., LABEL - .;		\
> -	FTR_ENTRY_LONG KEY;			\
> +	FTR_ENTRY_LONG KEY - .;			\
>  	.popsection
>  #endif
>  
> -- 
> 2.25.1
> 

Thanks for fixing the issue, Michael.

Perhaps the fix should go to v5.13-rc7 if Linus plans to do it. It'd be
good to avoid broken initrd on pseries guests in the release.

Regards,
Roman